Mimicking Ï Backdonation in Ce-MOFs for Solar-Driven Ammonia

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Mimicking # Backdonation in Ce-MOFs for Solar Driven Ammonia Synthesis Congmin Zhang, Yanling Xu, Chade Lv, Xin Zhou, Yu Wang, Weinan Xing, Qingqiang Meng, Yi Kong, and Gang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08682 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 24, 2019

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ACS Applied Materials & Interfaces

Mimicking π Backdonation in Ce-MOFs for Solar Driven Ammonia Synthesis Congmin Zhang,a Yanling Xu,a* Chade Lv,a Xin Zhou,a Yu Wang,a Weinan Xing,b Qingqiang Meng,a Yi Kong,a Gang Chen. a*

aDepartment

of Materials Chemistry, School of Chemistry and Chemical Engineering,

Harbin Institute of Technology, 150001, P. R. China. Email: [email protected]

bCollege

of Biology and the Environment,Nanjing Forestry University,

Nanjing,210037,China

KEYWORDS: Nitrogen fixation, mimicking π backdonation, cerium, MOFs.

ABSTRACT

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π backdonation is the core process to break through kinetically complex and energetically hurdle for catalyzing effectively NH3 synthesis, but only occurs on certain transition metals with empty and filled d orbits. Herein, mimicking π backdonation enables MOF-76(Ce) material to N2/NH3 conversion effectively. Note that by virtue of intrinsic mechanism of ligand metal charge transfer (LMCT), metal cerium species in MOF-76(Ce) serve as electrons sink for accumulating the photo-generated electrons. Taken together, experimental and theorical analysis reveal that such metal cerium with coordination unsaturated state (Ce-CUS) on MOF-76(Ce) nanorods surface also can provide unoccupied and occupied 4f orbits to accept from and then donate electrons back to nitrogen molecules. Remarkably, it shows outstanding photocatalytic nitrogen reduction performance with high average NH3 yield (34 mol×g-1×h-1) under ambient conditions. This work provides fresh insights into rational designing and engineering highly active catalysts with rare earth elements.

1. Introduction Ammonia (NH3) is an essential composition for fertilizer manmade and competitive candidate as carbon-free energy carriers in world energy storage field, hence its production expectation has 2 ACS Paragon Plus Environment

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been up to 200 million tonnes annually.(1-3) Inert dinitrogen (N2, 78% in the atmosphere) is largest resource for ammonia synthesis on earth, yet it is hard to convert into ammonia, due to the extremely high bond energy ( N≡N triple bonds, 941 kJ mol-1) and the large energy gap between the highest occupied and lowest unoccupied molecular orbits.(4, 5) Certain transition metals, for instance, Fe,(6) Ti,(7) Mo,(8) etc., have generally regarded as efficient active species for overcoming thermodynamic and kinetic hurdle in NH3 synthesis. This special ability arises from their unique and intriguing electronic structures with unoccupied and occupied d orbits, which are of appropriate energy and symmetry for accepting from and back-donating electrons to nitrogen molecules, termed as π backdonation. Recently, by virtue of the empty sp2 as well as filled p orbits at non-metal boron atom, Braunschweig’s group employed borylene fragment as a scaffold to achieve π backdonation imitation and nitrogen molecule functionalization, extending the corresponding range of catalyst onto main group element.(9) Therefore, mimicking such π backdonation process with analogous electronic structures that combine the empty and filled orbit proximal in energy and space, also enable valid nitrogen fixation on other elements.(10, 11) As the representative element among the rare earth family, cerium with [Xe] 4f26s2 electrons configuration shows the featured valence-variable behavior between Ce(III) and Ce(IV), equipped inherently with occupied 4f1 and unoccupied 4f0 orbits, respectively.(12, 13) Actually, benefiting from this charming electronic structure, Ce(III) salts as electrons donor and cerium(IV) ammonium nitrate as electrons acceptor have been already applied in organic chemistry widely.(14, 15) Conceivably, such flexible valence transformation allows fleet electrons acceptation and donation, making it an ideal catalyst to transiently coordinate with gas molecules in scalable photocatalytic transformations. Hereof, advances in chemistry of cerium



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element have pointed out that Ce sites furnished with the empty and filled orbits satisfy the requisite criteria on electrons structure for mimicking π backdonation. Over the past two decades, owing to the structural diversity, metal organic frameworks (MOFs), which were built from organic linker and metal nodes, emerged as a relatively new class porous coordination polymer materials and achieved the explosive development in catalysis.(16, 17) Complementary to the drawback of inorganic Ce-based semiconductor, Cebased MOFs materials deliver the following advantages for mimicking π backdonation. Chiefly, attributed to the flexible coordination circumstance of cerium metal nodes, abnormal cerium atom can be easily introduced and preserved in Ce-based MOFs materials to assist in π backdonation simulation.(18) Moreover, ligand to metal charge transfer (LMCT) would be favorable only in the unique case of Ce-based MOFs among multitudinous MOFs material, originated from the low-lying empty 4f orbits and the negative electron mobile energy (ELMCT).(19) It has been definitely indicated that photogenerated electrons would terminally accumulates on metal cerium center in recent report, which can act as active sites to participate in the subsequent π backdonation process. In light of these facts, rare earth metal cerium nodes in MOFs materials have great potential to achieve π backdonation simulation. To our best knowledge, however, no research steers from the exclusive view for mimicking typical π backdonation to realize nitrogen fixation on rare earth elements with 4f orbit configuration. Herein, we synthesized MOF-76(Ce) nanorods with a top-down mode, utilizing the wellknown nucleophilic attack of polar water molecules to dissociate the coordinated bonds between metal species and ligand. The surficial cerium species with coordination unsaturated state (CeCUS) on as-prepared MOF-76(Ce) play multiple roles in facilitating N2 activation and dissociation: (1). dominating the conduction band maximum of MOF-76(Ce) by theirs 4f orbits, 4 ACS Paragon Plus Environment

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which determines the electrons migration from ligand and gather on metal species. (2). mimicking π backdonation renders the easier and faster electron transfer to polarize N2 adsorbent. As a result, MOF-76(Ce) nanorods achieves an ammonia rate of 36.4 mol×g-1×h-1, 8 folds higher than that of the conventional cerium oxides. This work might open up a new vista for photocatalytic nitrogen fixation through mimicking π backdonation on rare earth elementsbased MOFs materials and reaffirm the versatility of metal CUS in tuning catalytic activity. 2. Experimental 2.1 Synthesis The preparation of precursor. Precursor was prepared via a facial one-step solvothermal method. Cerium nitrate hexahydrate Ce(NO3)3·6H2O, 217 mg, 0.5 mmol), benzene-1,3,5tricarboxylate (BTC, 210 mg, 1 mmol) were dissolved into 20 ml N, N-dimethylformamide (DMF). The mixture was treated by intense ultrasonic for 10 min and stirring for 30 min at ambient temperature. The resulting solution was transferred to a Teflon-lined stainless steel autoclave (25 mL) and kept at 120 °C for 48 h. Finally, the white product was collected, washed with deionized water and ethanol, and dried at 120 °C for 12 h. (20) The preparation of MOF-76(Ce) nanorods. A new defect induced nanofabrication strategy was applied prepare MOF-76(Ce) with nanostructure. The above MOF-76(Ce) product (0.2 mg) was added into 20 mL deionized water, followed by ultrasonic treatment for 10 min. Then light post-processing was conducted on the suspension for 30 min under UV-light irradiation by using a 300 W xenon lamp. The products were collected by centrifugation at 10000 rpm and washed several times with ethanol.



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The preparation of Ce-based oxides (Thermal processing sample). The MOF-76(Ce) precursor was treated at 500 °C in air for 2 h at a heating rate of 5 °C min-1. The calcinated sample was collected and named as CeO2. 2.2 Photocatalytic activity test A 200 mL quartz reactor was placed under the light source containing a mixture of 50 mg of the catalyst and 100 mL deionized water. The reaction mixture was stirred with N2 bubbling in the dark for 40 min to reach equilibration of N2 adsorption desorption on catalyst surface. The mixture was then irradiated with 300 W full spectra light source (Trusttech PLS-SXE 300, Beijing). Every 20 min of time intervals, 4 mL mixture was collected, followed by centrifugation to remove the catalyst particles. The supernatant was used to determine the concentration of ammonia by UV-vis spectroscopy based on the absorbance at 637 nm for ammonia solution with indophenol indicator. 2.3 Photoelectrochemical characterization The

photoelectrochemical

characteristics

were

measured

in

an

AUTOLAB-

PGSTAT302N electrochemical working station. And a standard three-compartment cell was placed under UV light provided by a 300 W Xe lamp. These samples coated at FTO glass, a piece of Pt sheet, an Ag/AgCl electrode dipping in the electrolyte of 0.5 M sodium sulfate were used as the working electrode, the counter-electrode and the reference electrode, respectively. 2.4 Characterization The samples crystal phases were characterized by powder X-ray diffractometer (XRD, RigakuD/max-2000) equipped with a Cu-K radiation in the 2 range of 10-90◦, scanning at 5◦


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min−1. Fourier transform infrared (FT-IR) spectra were studied with IR Affinity-1 spectrometer in the range of 500-4000 cm-1, using KBr pellets. Field-emission scanning electron microscopy (FESEM) was performed on the FEI QUANTA 200F microscope. The morphologies and microstructures of as-prepared composites were analyzed by transmission electron microscopy (TEM) (FEI Tecnai G2 S-Twin, accelerating voltage 300 kV). Thermal gravimetric-differential scanning calorimetry analysis (TG-DSC) was measured on SDT Q600 TG-DSC Instruments. The N2 adsorption-desorption isotherms were measured by an AUTOSORB-1-MP surface analyzer at 77 K. X-Ray photoelectron spectroscopy (XPS) was accomplished using a Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectrometer with a pass energy of 20.00 eV and an Al Kα excitation source (1486.6 eV). Diffuse reflectance spectra (DRS) data were recorded on a HITACHI UH4150 using BaSO4 as the reflectance standard sample. Fluorescence lifetime was measured using a lifetime spectrofluorometer (HORIBA Fluoromax-4) with a pulsed light-emitting diode (LED, λ= 301 nm) as the excitation light source. 2.5 Computational Details The calculations are based on DFT+U and employ the projected augmented wave (PAW) method,(21) as implemented in the Vienna ab initio simulation package.(22) In DFT+U, hubbard parameter of 4.50 eV was added for the Ce 4f states, which was used in previous first-principles calculations for cerium oxides.(23) A plane-wave cutoff energy of 400 eV were used for the total energy. The Brillouin-zone integrations were performed using a k mesh of 4×4×3 and 10×10×8 for the structure optimization and the following energy calculations, respectively. All energies and forces are optimized up to be smaller than 10-5 eV and 0.01 eV/Å, respectively. 3. Result and discussion



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Scheme 1. Schematic illustration of fabrication procedure for MOFs-76(Ce). MOF-76(Ce) formulated as {[Ce(BTC)(H2O)]×DMF}n, is composed of cerium mental centers and trimesic acid ligands (BTC). In this work, MOF-76(Ce) nanorods has been successfully prepared via solvothermal method in tandem with illumination treatment, as depicted in Scheme 1 (see synthetic details in the Experimental Section). Attributed to the success of typical method referenced from previous reports in the first stage,(20) the X-ray diffraction (XRD) peaks of assynthesized precursor microrods are well indexed with the crystal structure of MOF-76(Ce) phase (SEM and TEM images Figure S1a-c, XRD pattern Figure 1b). The successful preparation of well-ordered MOF-76(Ce) crystalline phase makes the followed size reduction process induced by coordinated bond dissociation possible. More specifically in step two, the coordination environments of cerium metal nodes are drastically changed, due to the nucleophilic attack of polar water molecular and the removal of solvent molecules under photoprocessing. Thus, the coordination interactions between cerium atoms and the carboxyl oxygen in BTC are impaired even cut off, rendering the size of sample down into nano-scale. As a consequence, numerous cerium coordination unsaturated sites (Ce-CUS) are gradually exposed 8 ACS Paragon Plus Environment

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on fracture surface. The corresponding size fragment process along with the cracks horizontal and vertical propagation in 3D frameworks is recorded by the morphology evolution images (Figure S2).

Figure 1. MOF-76(Ce) structure characterization and schematic diagram: (a) TEM image of MOF-76(Ce) nanorods, (b) XRD patterns and (c) FT-IR spectra of precursor and MOF-76(Ce), (d) Cerium metal node and BTC ligand as structure component, oxygen atoms in carboxyl group and H2O are present in pink and blue. (e) View on 3D periodic framework of {[Ce(BTC)(H2O)]·DMF}n along C axis, (f) 1D sinusoidally-shaped channels viewing from A



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axis and one-dimensional left-handed helical chains built from CeO8 polyhedra. XPS fine spectra of (g) Ce 4f, (h) O 1s and (i) C 1s. The morphology and structure of terminal MOF-76(Ce) product are expounded as below. Scanning electron microscope (SEM) images reveal the highly uniform nano-scale rod-like morphology of MOF-76(Ce) (Figure S5a). A closer inspection at transmission electron microscope (TEM) images show the its diameter size down to » 70 nm (Figure 1a, also Figure S5b, c). Besides, MOF-76(Ce) nanorods exhibit a low specific area of 13.87 m2×g-1 arising from the channel structure distortion and dense pseudo-polymorph translation, which is induced by the ordered remove of coordination water and solvent guest molecules in 3D MOF-76(Ce) frameworks at 120 ℃ synthesized temperature.(24) However, originated from the prominent size decrease, it is still more than 2 fold compared to that of precursor (5.05 m2×g-1) (Figure S6, N2 adsorption-desorption isotherm). For same reason, X-ray diffraction (XRD) patterns show the diagonal contraction and distortion of the channels arouse the peaks at 18° belonging to (010) to slightly shifts towards higher angles, due to the partial removal of solvent molecules. But the crystalline degree of broad humps is too low to uncover the inner structure (Figure 1b). Circumventing this barrier, fourier transform infrared spectroscopy (FTIR) is performed to further ascertain the unit structure of MOF-76(Ce) (Figure 1c). For MOF-76(Ce), the usual peaks of carboxylate groups in BTC3- ligands are observed, presenting as the asymmetric vibration modes (uas (COO-)) at 1612 cm-1 and symmetric vibrations modes (us (COO-)) 1382 cm-1. Simultaneously, the characteristic bands at 500−700 cm−1 are assigned to Ce−O stretching vibrations, which demonstrates cerium metal ions indeed coordinate with BTC3- ligand to construct the 3D frameworks of MOF-76(Ce). It is noteworthy that the inner structure of MOF76(Ce) nanorods have no difference from precursor according to their similar spectra. TG curve 10 ACS Paragon Plus Environment

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of precursor also confirms the stability of MOF-76(Ce) at the synthesized temperature in this work (Figure S7). The collapse of MOF-76(Ce) framework starts until 350 °C because of the fast pyrolysis of organic ligand. Despite a little mass loss caused by volatile solvent guest molecule and coordinated H2O in 100-200 °C, the intrinsic constitution units of MOF-76(Ce) in terminal product are inherited from the precursor. (25) The specific coordination environment of cerium metal center is described as Figure 1d. There are three deprotonated carboxyl groups in per BTC3- ligand, one in synsyn mode and two in chelating-anti mode coordinates to cerium metal center. For the latter, one of oxygen atoms in carboxyl groups can only offer single electron to develop a semi-coordination state, due to the unique steric configuration of chelating-anti coordination. Whereupon, besides one oxygen atom from water molecule, 4f orbits of Ce(III) are totally occupied by three semi-coordinated and four usual oxygen atoms of BTC3- ligand, and thus constructing eight donor set around each of Ce(III) ions, donated as CeO8 polyhedron with distorted bicapped trigonal prism configuration. (26) Such CeO8 clusters locates at the nodes in 3D frameworks of tetragonal non-centrosymmetric chiral space group P43, establishing the lefthanded helical chains of CeO8 polyhedra propagating along the crystallographic screw 43 axis (Figure 1e, f). However, when this crystal size decreases into nanoscale, a large number of cerium ions are exposed on fracture surface and convert into Ce coordination unsaturated state (Ce-CUS), consequently offering the unoccupied and occupied 4f orbits for π electrons backdonation to N2 molecules. To validate such π electrons backdonation process on coordination unsaturated cerium sites, X-ray photoelectron spectroscopy (XPS, Figure 1g-i) was indispensably implemented to track Ce-CUS. High-resolution core level spectrum for Ce 4f signals of MOF-76(Ce) presents the deconvolution into two Voigt doublets corresponding to Ce(III), labeled as (u0 + v0) and (u’+ 11 ACS Paragon Plus Environment


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v’). Notably, as expected, one Voigt doublet seats closer to the literature value of fitting Ce(IV) parameters (u’’+ v’’), this is because the unsaturated coordination environment maximizes surficial cerium sites to a higher binding energy(BE) forming the Ce-CSU like Ce(IV) oxidation state. As the characteristic peak of Ce(IV), the u’’’ peak is originated from the shake down transition of ground state excitation (4f0-4f1 L-1, L-1 is a valence-band hole). But MOF-76(Ce) would not exhibit the u’’’ peak at highest binding energy (916 eV), indicating Ce-CUS maintain 4f1 orbits and unique unsaturated environment differing from either Ce(III) or Ce(IV). For quantifing the data used in the following equation, all the collected Ce 4f spectra were curve fitted and area integrated, yielding a relative amount of ≈10-15% abnormal Ce-CSU. (27) Concentration of abnormal Ce = å Peak area (abnormal Ce)/ å Peak area (total Ce)


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Figure 2. Theoretical calculation to demonstrate π electrons backdonation: total and projected density of state for MOF-76(Ce) without (a) or with (b) N2 adsorption, (c) optimal structure for adsorption geometry of N2 on Ce-CUS. Band structure: (d) Mott-Schottky plots, (e) XPS Valence band spectra, enlarged figure with binding energy range from -1 to 2 eV are in the inset. (f) UV-vis Diffused Reflectance Spectra (DRS), the inset shows corresponding Tauc plots based on the Butler equation of the plots of (ah)2 versus h. (g) Schematic diagram for the energy band structure and Ce-CUS donor level. (h) Transient photocurrent responses bubbling with N2 and Ar gas. (i) Time-resolved fluorescence decay curves of MOF-76(Ce) and CeO2. Both sides support this view, an amount of adsorbed water molecules covers Ce-CSU easily, giving rise to high hydroxyl peaks at 531.5 eV as observed from Figure 1h of O 1s. The chemical environment of carbon atom far from Ce-CSU have no extraordinary phenomenon, except for the peaks of aromatic ring (C-C, 284.2 eV and C=C, 285.2 eV) and carboxyl (O-C=O, 288.2 eV) in BTC linker (Figure 1i). (28) To clarify the rational deduction that Ce-CUS would act as active sites and mimic π electrons backdonation to N2 molecules in photocatalysis, an involved interaction between CeCUS and N2 molecules are essentially confirmed by a combination of theoretical calculation and precision experiment. Periodic Density Functional Theory (DFT) calculations reveal the valence band (VB) and conduction band (CB) are mainly dominated by O 2p, C2p and Ce 4f orbits, respectively (Figure 2a). Particularly, the increased Ce 4f orbit prominently broadens and shifts towards the low-energy direction with the introduction of Ce-CUS by exposing MOF-76(Ce) cross-section, resulting in the narrow band gap (Figure 2b). Through the fitted analysis of DOS contribution on local cerium atom, the exceptional donor level is mainly composed of surficial



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Ce-CUS. It indicates that the linker should be responsible for light absorption and excitation, (Liner-localized excitations), then electrons would be further transferred to the lowest unoccupied orbit of Ce-CUS, known as ligand-to-metal charge transfer (LMCT). This spontaneous process with negative LMCT energy in MOF-76(Ce) renders electrons accumulation favorable over Ce-CUS. (19) Furthermore, owing to the low-lying nature of CeCUS empty 4f orbit, the density of state on the crystal model with N2 adsorption reflects the effective overlap of cerium 4f orbit and N2 molecule. This result directly verify the fact that the empty 4f orbit of Ce-CUS can accept electrons from σ orbit of N2, and then highly reducible Ce(III) back-donate electrons to π* antibonding orbit of N2. Attributed to such π backdonation process, N≡N triple bonds would be weaken and elongated to 1.117 Å (Figure 2c), which is intermediary between the triple bond length (1.078 Å) of free molecular nitrogen and the double bond length (1.201 Å) of diazene. (29) Thus, it is definite that Ce-CUS can play the significant role as active sites to mimic π backdonation process, facilitating the activation and transformation for N2 molecules. To consolidate these theoretical calculation results of electrons accumulation and N2 activation on Ce-CUS, experimental study on band structure is necessary. Almost no shift occurs on VB verified by XPS Valence band spectra (Figure 2e). But the plots Mott-schottky analysis verify that the flat band potential (Efb) of MOF-76(Ce) nano-rods with n-type semiconductor feature decline 0.5 eV comparing to that of precursor (Figure 2d). This is attributed to the contribution of lower lying 4f orbits of Ce-CUS for CB moving down, and it would narrows band gap consistent with calculation results. As expected, UV-vis Diffused Reflectance Spectra (DRS) shows the band gap of MOF-76 nanorods decrease from 3.25 eV to 3.2 eV along with Ce-


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CUS appearance (Figure 2f). The corresponding illustration of band structure before and after the introduction of Ce-CUS are displayed in Figure 2g. In addition, the fluorescence lifetime decay curves fitted in a double-exponential model was employed to testify LMCT process that can promote electrons transfer and terminal accumulation on Ce-CUS (Figure 2h). Two emissive state belonging to interband exciton recombination and eletron-hole recombination are corresponding to the decays in MOF-76(Ce) with τ1 = 2.2 ns (A1, 33.07%) and τ2 = 16.05 ns (A2, 66.93%). Its average lifetime τ is 15.17 ns so higher than that of cerium oxides CeO2 (τ1, 0.8 ns; A1, 0.02%; τ2 5.1 ns; A2, 99.98%; τ, 3.94 ns), successfully attesting the MOFs structure-dependent long distance eletrons transfer from organic linker to cerium nodes in LMCT process. Photoluminescence spectra also shows the excellent carriers separation efficiency of MOF-76(Ce) due to LMCT process (Figure S8). A large amount of photogenerated electrons would concentrate on the 4f orbits of Ce-CUS which dominant the CB bottom, then transfer towards N2 via π backdonation. As a corroborative evidence for above process, photocurrent response of MOF-76(Ce) blowing N2 is merely 1/2 of that under Ar atmosphere (Figure 2i). Because the high energy electrons on Ce-CUS can be rapidly donated to reduce N2 molecules rather than to form electricity in electrolyte, indicative of the strong interaction between Ce-CUS and N2 on account of π backdonation.



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Figure 3. Photocatalytic nitrogen fixation over MOF-76(Ce) comparing to CeO2: (a) plots of NH3 release as a function of time, (b) cationic liquid chromatograms of NH4+ before and after full-spectra illumination, (c) UV–Vis absorption spectra of sample liquid extracted at different reaction time and stained with indophenol indicator (d) contract test under condition of Ar gas bubbling and CH3CN solvent. The above-mentioned results and discussion provided solid evidence that Ce-CUS can play a significant role as active sites to dissociate N2 molecules. Encouraged by these, the classical indophenol colorimetry is used to determine NH3 yield (Corresponding standard curve and instrument photo are displayed in Figure S9). (7, 30) Figure 3a compares the amounts of NH3 generated on MOF-76(Ce) and CeO2, which both synthesized from same precursor. MOF76(Ce) exhibit much better photocatalytic activity as high as 34.2 mol×g-1×h-1 than CeO2, and the concentration of NH3 stably increased along with time. The excellent photocatalytic nitrogen fixation activity of MOF-76(Ce) can be attributed to the π backdonation on Ce-CUS. In stark 16 ACS Paragon Plus Environment

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contrast, even if CeO2 also contains cerium ingredient, owing to crystal integrity, it cannot provide the Ce-CUS with the unoccupied and occupied 4f orbits to efficiently mimic π backdonation and cleave N≡N triple bond. Besides, there is no noticeable change in the NH3production rate for MOF-76(Ce) during the four catalytic runs, suggesting its favorable stability for nitrogen fixation. The corresponding UV–vis absorption spectra of liquid sample extracted at different irradiation time are presented in Figure 3c. To corroborate NH3 generation, ions chromatography was carried out to detect amounts of NH3 in photocatalytic system. As seen in Figure 3b, MOF-76(Ce) nanorods sample exhibits doublets correspond to NH4+ after 1 h irradiation comparing to that in dark, and the NH4+ yield of 36.4 mol×g-1×h-1 coincides well with the value estimated in indophenol method. In addition, in situ FTIR spectra was performed to detect NH3 production during photocatalytic nitrogen fixation in real-time. The peaks area of NH3 (1190 and 1300 cm-1) and NH4+ (2870 cm-1) gradually increased along with extending irradiation time, directly verify the effective conversion from N2 to NH3 (Figure S10). Note that the efficiency of MOF-76(Ce) is still relatively splendid among virous catalyst (Table S1), and finding is encouraging for further improvement in the future. Controlled experiment in case of pumping with Ar, little NH3 was traced because of the obstruction of N2 (Figure 3d). When using an aprotic solvent (CH3CN) instead of water, the drastically decreased activities was observed, indicating that water offer proton for NH3 synthesis, as well as proved by the ability of MOF76(Ce) for water splitting (Figure S11). (31) Accordingly, π backdonation simulation on Ce unsaturated sites promote the photocatalytic NH3 evolution from N2 and water. To eliminating the influence of NO3- in Ce(NO3)3 for nitrogen fixation, the MOF-76(Ce) prepared by CeCl3 was used as catalyst and detected nitrogen fixation performance (Figure S12). Its NH3 yield rate was as high as that of sample originated from CeNO3, clearly prove NO3- existence in raw ingredient



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have little influence on NH3 synthesis. To evaluate the catalyst selectivity for NH3, we adopted the typical Watt and Chrisp method to detect N2H4. Corresponding standard curve: y = 0.0060x + 0.0065 is displayed in Figure S13. The average yield of N2H4 is only 5.2 umol·g-1·h-1, which is much lower than that of NH3. This indicates NH3 is the major production and MOF-76(Ce) own a good selectivity for NH3 synthesis. Moreover, the competition of NH3 and H2 synthesis are also investigate in Figure S14. In addition, evaluated by a probe photocatalytic reaction with MOF76(Ce) catalyst for methylene blue (MB) degradation, electron reduced superoxide anions (O2-) were main active species in both condition of dye-sensitizing visible and full spectra irradiation, indicating the high ability of photogeneratd electrons reduction on MOF-76(Ce) (Figure S15). 4. Conclusion Nanorods-like MOF-76(Ce) materials containing Ce-CUS on surface is successfully fabricated with a top-down mode. This Ce-CUS dominants the bottom of CB and collects high energy electrons through the typical LMCT process. Moreover, it owns inherent empty and filled 4f orbits to provide low-lying donor level, promptly accepting from and back-donating electrons to N2, thus acting as abundant active sites to effectively cleave N2 molecules. Such π backdonation stimulation render MOF-76(Ce) standout in photocatalytic nitrogen fixation. And this work may open up novel ideas for designing and engineering the catalyst of nitrogen fixation with rare earth elements. ASSOCIATED CONTENT


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Corresponding Supporting Information (SI) provides details of synthetic procedures and materials characterization method. In addition, this SI contains TEM images and TG curve of precursor; morphology revolution mechanism; XRD pattern, FTIR spectrum and SEM image of contrast CeO2 sample; morphology images, N2 adsorption–desorption isotherm, Photoluminescence spectra, Schematic diagram for the energy band structure, Standard curve of indophenol blue colorimetry, Comparison of N2 photocatalytic reduction activity, Photocatalytic performance for hydrogen and oxygen evolution. AUTHOR INFORMATION

Corresponding author: E-mail: [email protected], Fax: (+86)-451-86413753 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21471040) and (21403046).

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